Prodigal son Mark Gasnier scored the match-winner as St George Illawarra powered home to a 16-13 NRL win over South Sydney at ANZ Stadium on Friday night.

Gasnier was criticised for his NRL comeback from French rugby against Penrith two weeks ago, but showed why the Dragons went to so much trouble to get him back, by coming up with two clinching plays against the Rabbitohs.

With his team trailing 13-12 after a Chris Sandow field goal, Gasnier sprinted more than 20 metres for his first NRL try in almost two years to give the Dragons a boosting win over spirited opposition.

On the last tackle with just nine minutes left on the clock, Queensland second rower Neville Costigan sparked the winning-play out of the unfamiliar position of dummy-half.

Costigan dummied and went the blind-side, seeing Kiwi flyer Jason Nightingale down the right-flank before he drew fullback Rhys Wesser and sent Gasnier on his way.

The Dragons trailed 12-4 after a John Sutton try early in the second half, and it was a Gasnier pass to Nightingale, for the winger to reach out and score in the corner, which got the competition leaders back into the game.

With the match as evenly contested as any this season, the Dragons took a chance to draw level through a Jamie Soward penalty kick in 58th minute after Sam Burgess was penalised for stripping Trent Merrin.

Even with 16 minutes still remaining on the clock, Sandow decided his team might as well play from in front and potted a 30 metre field goal to give the Bunnies the lead.

In the end though it wasn't enough, and Souths were left heart-broken after a second last-gasp loss in as many weeks.

South Sydney had held 6-4 lead at halftime, after dominating most of the opening 40 minutes.

The home side opened the scoring in the fifth minute when captain Roy Asotasi strolled through some weak defence.

St George Illawarra finally hit back in the 27th minute, after a trademark backline play instigated by a Darius Boyd cut out ball, finished with a diving Brett Morris effort in the left corner.

St George Illawarra captain Ben Hornby said Gasnier was delighted to have scored the winning try.

"Obviously he copped a bit of stick last week but he's going to get better," Hornby said.

"He's still way off where he can be but he's going to get better every week so he's over the moon to get a try that's for sure."

Dragons coach Wayne Bennett said the match was won in the first half, when their defence conceded just one try to the rampaging Rabbitohs.

"That didn't worry me too much. It was a long way out from the end of the game, it wasn't the last three minutes," Bennett said of Sandow's field goal.

"I think that's (defence in first half) where we won it, that's what was crucial.

"We did what we did, it kept us in the game, gave us a bit of confidence.

"We come in at halftime knowing we'd been outplayed pretty well but we were still in there and knew we would be better in second half.

"I thought we were, and we made it a contest in the second half and got away with it."

Rabbitohs coach John Lang said the fact his team hasn't spoken out about poor refereeing this season has cost his side.

"It's very disappointing. I thought we got some really rough calls," Lang ranted.

"I've said nothing all year about the referees, I thought we got some really rough calls from the ref tonight."

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